1. Field of the Invention
This invention relates to audio transducer compensation, and more particularly to a method of compensating non-linear distortion of an audio transducer such as a speaker, earphone or microphone.
2. Description of the Related Art
Audio transducers preferably exhibit a uniform and predictable input/output (I/O) response characteristic. In a speaker, the analog audio signal coupled to the input of a speaker is what is ideally provided at the ear of the listener. In reality, the audio signal that reaches the listener's ear is the original audio signal plus some distortion caused by the speaker itself (e.g., its construction and the interaction of the components within it) and by the listening environment (e.g., the location of the listener, the acoustic characteristics of the room, etc) in which the audio signal must travel to reach the listener's ear. There are many techniques performed during the manufacture of the speaker to minimize the distortion caused by the speaker itself so as to provide the desired speaker response. In addition, there are techniques for mechanically hand-tuning the speaker to further reduce distortion.
Distortion includes both linear and non-linear components. Non-linear distortion such as “clipping” is a function of the amplitude of the input audio signal whereas linear distortion is not. Klippel et al, ‘Loudspeaker Nonlinearities—Causes, Parameters, Symptoms’ AES Oct. 7-10, 2005 describes the relationship between non-linear distortion measurement and nonlinearities which are the physical causes for signal distortion in speakers and other transducers.
There are many approaches to solve the linear part of the problem. The simplest method is an equalizer that provides a bank of bandpass filters with independent gain control. Techniques for compensating non-linear distortion are less developed.
Bard et al “Compensation of nonlinearities of horn loudspeakers”, AES Oct. 7-10, 2005 uses an inverse transform based on frequency-domain Volterra kernels to estimate the nonlinearity of the speaker. The inversion is obtained by analytically calculating the inverted Volterra kernels from forward frequency domain kernels. This approach is good for stationary signals (e.g. a set of sinusoids) but significant nonlinearity may occur in transient non-stationary regions of the audio signal.